DISCLOSURES ABOUT THE FAIR VALUE OF FINANCIAL INSTRUMENTS, Recurring Measurement Inputs (Details) $ in Thousands	Mar. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 1,256,473	$ 1,275,016
Equity investments, at fair value	220,498
Equity securities available-for-sale		228,115
Short-term investments	21,664	23,613
Liabilities [Abstract]
Surplus notes	25,000	25,000
Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	1,256,473	1,275,016
US Treasury Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	7,956	8,078
US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	302,476	297,949
US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	293,931	307,536
Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	76,660	83,980
Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	132,350	119,799
Collateralized Debt Obligations [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	22,748	24,114
Corporate Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	420,352	433,560
Equity Securities [Member]
Assets [Abstract]
Equity securities available-for-sale		228,115
Common stocks Financial services [Member]
Assets [Abstract]
Equity securities available-for-sale		43,522
Common stocks Information technology [Member]
Assets [Abstract]
Equity securities available-for-sale		35,810
Common stocks Healthcare [Member]
Assets [Abstract]
Equity securities available-for-sale		30,595
Common stocks Consumer staples [Member]
Assets [Abstract]
Equity securities available-for-sale		14,127
Common stocks Consumer discretionary [Member]
Assets [Abstract]
Equity securities available-for-sale		20,538
Common stocks Energy [Member]
Assets [Abstract]
Equity securities available-for-sale		16,905
Common stocks Industrials [Member]
Assets [Abstract]
Equity securities available-for-sale		28,489
Common stocks Other [Member]
Assets [Abstract]
Equity securities available-for-sale		16,421
Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Equity securities available-for-sale		21,708
Recurring [Member]
Assets [Abstract]
Short-term investments	21,664	23,613
Recurring [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	1,256,473	1,275,016
Recurring [Member] | US Treasury Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	7,956	8,078
Recurring [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	302,476	297,949
Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	293,931	307,536
Recurring [Member] | Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	76,660	83,980
Recurring [Member] | Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	132,350	119,799
Recurring [Member] | Collateralized Debt Obligations [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	22,748	24,114
Recurring [Member] | Corporate Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	420,352	433,560
Recurring [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	220,498
Equity securities available-for-sale		228,115
Recurring [Member] | Common stocks Financial services [Member]
Assets [Abstract]
Equity investments, at fair value	44,895
Equity securities available-for-sale		43,522
Recurring [Member] | Common stocks Information technology [Member]
Assets [Abstract]
Equity investments, at fair value	33,886
Equity securities available-for-sale		35,810
Recurring [Member] | Common stocks Healthcare [Member]
Assets [Abstract]
Equity investments, at fair value	28,372
Equity securities available-for-sale		30,595
Recurring [Member] | Common stocks Consumer staples [Member]
Assets [Abstract]
Equity investments, at fair value	14,715
Equity securities available-for-sale		14,127
Recurring [Member] | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Equity investments, at fair value	21,966
Equity securities available-for-sale		20,538
Recurring [Member] | Common stocks Energy [Member]
Assets [Abstract]
Equity investments, at fair value	15,620
Equity securities available-for-sale		16,905
Recurring [Member] | Common stocks Industrials [Member]
Assets [Abstract]
Equity investments, at fair value	25,235
Equity securities available-for-sale		28,489
Recurring [Member] | Common stocks Other [Member]
Assets [Abstract]
Equity investments, at fair value	15,762
Equity securities available-for-sale		16,421
Recurring [Member] | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Equity investments, at fair value	19,224
Equity securities available-for-sale		21,708
Recurring [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	823
Recurring [Member] | Investments measured at net asset value (NAV)
Assets [Abstract]
Short-term investments	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | US Treasury Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Collateralized Debt Obligations [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Corporate Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	823
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Common stocks Financial services [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Common stocks Information technology [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Common stocks Healthcare [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Common stocks Consumer staples [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Common stocks Energy [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Common stocks Industrials [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Common stocks Other [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	823
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets [Abstract]
Short-term investments	21,664	23,613
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | US Treasury Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Collateralized Debt Obligations [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Corporate Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	209,891
Equity securities available-for-sale		215,916
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Common stocks Financial services [Member]
Assets [Abstract]
Equity investments, at fair value	44,892
Equity securities available-for-sale		43,519
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Common stocks Information technology [Member]
Assets [Abstract]
Equity investments, at fair value	33,886
Equity securities available-for-sale		35,810
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Common stocks Healthcare [Member]
Assets [Abstract]
Equity investments, at fair value	28,372
Equity securities available-for-sale		30,595
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Common stocks Consumer staples [Member]
Assets [Abstract]
Equity investments, at fair value	14,715
Equity securities available-for-sale		14,127
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Equity investments, at fair value	21,966
Equity securities available-for-sale		20,538
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Common stocks Energy [Member]
Assets [Abstract]
Equity investments, at fair value	15,620
Equity securities available-for-sale		16,905
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Common stocks Industrials [Member]
Assets [Abstract]
Equity investments, at fair value	25,235
Equity securities available-for-sale		28,489
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Common stocks Other [Member]
Assets [Abstract]
Equity investments, at fair value	15,762
Equity securities available-for-sale		16,421
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Equity investments, at fair value	9,443
Equity securities available-for-sale		9,512
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	0
Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Assets [Abstract]
Short-term investments	0	0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	1,255,911	1,274,396
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | US Treasury Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	7,956	8,078
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	302,476	297,949
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	293,931	307,536
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	76,660	83,980
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	132,350	119,799
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Collateralized Debt Obligations [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	22,748	24,114
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	419,790	432,940
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	9,781
Equity securities available-for-sale		10,196
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Common stocks Financial services [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Common stocks Information technology [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Common stocks Healthcare [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Common stocks Consumer staples [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Common stocks Energy [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Common stocks Industrials [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Common stocks Other [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Equity investments, at fair value	9,781
Equity securities available-for-sale		10,196
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Assets [Abstract]
Short-term investments	0	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 562	$ 620
Number of securities categorized as Level 3 fair value measurements | Security	2	2
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | US Treasury Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 0	$ 0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Collateralized Debt Obligations [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Corporate Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	562	620
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	$ 3
Equity securities available-for-sale		$ 2,003
Number of securities categorized as Level 3 fair value measurements | Security	1	2
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Common stocks Financial services [Member]
Assets [Abstract]
Equity investments, at fair value	$ 3
Equity securities available-for-sale		$ 3
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Common stocks Information technology [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Common stocks Healthcare [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Common stocks Consumer staples [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Common stocks Energy [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Common stocks Industrials [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Common stocks Other [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		2,000
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	0
Not Reported At Fair Value [Member]
Liabilities [Abstract]
Surplus notes	15,731	16,689
Not Reported At Fair Value [Member] | Investments measured at net asset value (NAV)
Liabilities [Abstract]
Surplus notes	0	0
Not Reported At Fair Value [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Liabilities [Abstract]
Surplus notes	0	0
Not Reported At Fair Value [Member] | Significant other observable inputs (Level 2) [Member]
Liabilities [Abstract]
Surplus notes	0	0
Not Reported At Fair Value [Member] | Significant unobservable inputs (Level 3) [Member]
Liabilities [Abstract]
Surplus notes	$ 15,731	$ 16,689